The document U.S. Pat. No. 5,184,510 A discloses an apparatus and a method for measuring the level of a liquid within a storage tank, including a target for being buoyantly supported near the surface of the liquid and an acoustic sensor for measuring the distance between a tank reference point and the target.
The document U.S. Pat. No. 6,427,532 A discloses a device for measuring a fill level of a liquid includes an ultrasonic sensor with two sensor elements. One of the sensor elements determines the echo time of an ultrasonic wave to a float which follows the liquid level. The other of the sensor elements determines the echo time of the ultrasonic wave up to a measuring reflector arranged at a known distance from the other of the sensor elements. An evaluation unit compares the echo times and determines the fill level of the liquid from the ratio of echo times and from the known distance to the measuring reflector.
The document DE 10 2005 035 846 A1 discloses a device for measuring a fill level comprising a receiving unit floating on a surface of a liquid which uses distance measurement based on signals emitted by an emitting unit to measure the distance between the emitting and receiving unit to determine the fill level.
Measurement of fill levels of liquids in containers, tanks, vessels or other reservoirs containing liquids in aircraft or spacecraft poses certain challenges due to the generally adverse environmental conditions. In waste tanks, potable water tanks or fuel tanks the amount of contained liquids and possibly other objects has to be determined under variable cabin and ambient pressure.